This invention relates to digital mobile phone systems (e.g. GSM (global system for mobile communication) phones).
Digital mobile phone systems (e.g. GDM phones) rely for their operation on accurate matching of a local timebase within a mobile station with the timebase of a base station. During idle or standby operation, the mobile phone must receive pages periodically, together with decoding broadcast information concerning the network configuration. For power-saving between these periods of activity it is desirable to shut down as much of the mobile station circuitry as possible and, as timebse circuitry operates at a high frequency, for example 13 MHz in the GSM system, considerable power can be saved by shutting down the timebase circuitry "sleep" periods.
GB 2297884 A discloses a power saving arrangement in a mobile telephone which has a high frequency system clock and a processor arranged to process polling signals received while the telephone is in its standby condition. When polling signals are not being received, it is possible for the telephone to be placed in a sleep condition, by de-activating the system clock. Re-activation occurs in response to a calibrated number of clock cycles produced by a lower frequency sleep clock. Upon re-activation, system clock counters, specifying sub-frame periods and frame periods are re-loaded so that they may be re-activated at the required phase. The phase of these counters is compared with signals received from base stations and modifications are made to system counts as required. The extent to which modification are required is also used to re-calibrate the sleep clock.
EP 0586256 A2 discloses a time measurement system for measuring time accurately with an inaccurate clock. Two clock oscillators are compared and the momentary error of the slower clock oscillator is measured. When the error change rate of the slower clock oscillator is slow enough, the fast clock oscillator can be switched off for longer time interval. With the help of this apparatus and method of operation power can be saved in portable equipment which requires accurate time measurement.
However, re-synchronizing the mobile station timebase with the base station timebase on re-awakening (e.g. for paging reception) needs to be accomplished quickly and it is therefore highly desirable to reconstruct the mobile station timebase in synchronizm with its operation before shutdown without reference to the base station signals, or utilizing the base station signals to reconstruct the time base takes time and uses considerable power.